The ability to discriminate between exposed and unexposed areas of film or paper is the most basic requirement of any photographic recording device. In a normal sequence, the exposed photographic element is subjected to a chemical developer, wherein a very large amplification is effected through production of metallic silver as a result of catalytic action of small latent image centers that are believed to be small silver or silver and gold clusters. The resulting silver then forms the final image in many black and white products, or oxidized developer resulting from the silver reduction reaction can be reacted with couplers to form image dye. In either case, because of the thermodynamic driving force of the chemical developer to reduce silver halide to silver, it is not surprising that achievement of the desired discrimination between exposed and unexposed regions of a photographic element continues to challenge photographic scientists: Any non-image catalytic center will facilitate the unwanted production of metallic silver and image dye in unexposed areas during the development process. These non-image catalytic centers can come from one or more of various sources; for example, they may be the result of an inadvertant reductive process that generates Ag centers, they may be silver sulfide or silver/gold sulfide centers that result from inadvertant oversensitization, or they may result from trace metals such as iron, lead, tin, copper, nickel and the like from raw materials and/or manufacturing equipment.
Because there can be a variety of causes of photographic fog, a number of methods have been devised to combat it. One approach is to add one or more oxidants at various stages of the manufacturing process. Such oxidants include, for example, hydrogen peroxide or precursors of it, halogen or halogen releasing compounds, mercuric ion, or dichalcogenides such as bis(p-acetimidophenyl) disulfide (U.S. Pat. No. 5,219,721--Klaus or European Patent Application 0 566 074 A2--Kim). Selected oxidants are especially useful in minimizing reductive type fog.
A second approach involves addition of organic materials that tightly adsorb to the surfaces of silver halide light sensitive crystals, often through formation of sparingly soluble adducts with silver ion. Commonly used materials include, for example, tetraazaindenes (Carroll et al U.S. Pat. No. 2,716,062), benzothiazoliums (Brooker et al U.S. Pat. No. 2,131,038; Allen U.S. Pat. No. 2,694,716), or mercaptotetrazoles (Abbott et al U.S. Pat. No. 3,295,976; Luckey U.S. Pat. No. 3,397,987). While such materials can minimize reduction of silver halide to silver by reducing the silver ion concentration, they are also presumed to block those portions of the AgX surface to which they are adsorbed, thereby arresting the chemical sensitization and preventing the buildup of silver sulfide or silver gold sulfide centers to a size that allows them to become capable of catalyzing the silver development process.
A third approach utilizes complexing agents that are presumed to sequester metals, thereby mitigating their fogging propensity. Such agents include, for example, sulfocatechol-type materials (Kenard et al, U.S. Pat. No. 3,236,652), aldoximes (Carroll et al, U.K. Patent 623,448), meta and poly-phosphates (Draisbach, U.S. Pat. No. 2,239,284), carboxyacids (U.K. Patent 691,715) or sulfo-salicyclic acid type compounds (Willems, U.S. Pat. No. 3,300,312).
In recent years, the utility of tabular grain emulsions has become evident following disclosures of Kofron et al (U.S. Pat. No. 4,439,520). An early cross-referenced variation on the teachings of Kofron et al was provided by Maskasky (U.S. Pat. No. 4,435,501) Maskasky demonstrated significant increases in photographic sensitivity as a result of selected site sensitizations involving silver salt epitaxy. Still more recently, Antoniades et al (U.S. Pat. No. 5,250,403) taught the use of ultrathin tabular grain emulsions in which the tabular grains have an equivalent circular diameter (ECD) of at least 0.7 .mu.m and a mean thickness of less than 0.07 .mu.m, and in which tabular grains account for greater than 97 percent of the total grain projected area. Kodak patent applications now on file teach epitaxial sensitization of ultrathin tabular emulsions in which the host and epitaxy have preferred composition or dopant management (Daubendiek et al U.S. Ser. Nos. 296,841; 297,430 [Daubendiek II]); and 297,195 all filed Aug. 26, 1994, and Olm et al U.S. Ser. No. 296,562 filed Aug. 26, 1994).
Epitaxially sensitized emulsions in general, and epitaxially sensitized ultrathin tabular emulsions in particular, present some unique challenges in selection of antifoggants. This is due to the presence of at least two different silver salt compositions in the same emulsion grains. Thus in the case of Ag(Br, I) hosts that have AgCl-containing epitaxy deposited on them, it is not immediately evident whether addenda should be selected that are appropriate to the Ag(Br, I) host or to the AgCl-containing epitaxy. It is further complicated by the fact that the host and epitaxy will likely have different exposed crystal lattice planes, and what adsorbs to host planes may not adsorb to those of the epitaxy, or an addendum that stablizes one surface may destabilize the other. Moreover, there is a strong entropic driving force for the Ag(Br, I) host and AgCl regions to recrystallize to form a single uniform composition (C. R. Berry in The Theory of the Photographic Process, 4th Ed., T. H. James, Ed., New York: Macmillan Publishing Co., Inc., (1977), p 94f). Finally, if the Ag(Br, I) host is ultrathin, there is the additional strong tendency for Ostwald ripening to occur due to the high surface energy resulting from their large surface area/volume ratio (C. R. Berry, loc cit, p 93). For these reasons, choice of antifogging addenda for epitaxially sensitized tabular grain emulsions is not at all obvious.
Finally it is important to note that while discrimination between exposed and nonexposed areas is the most basic requirement of a photographic film or paper, it is by no means the only one. In particular, it is highly desirable to achieve stabilization against fog without degradation of sensitivity, developability, or image structure. The most preferable method would minimize fog, increase photographic speed, and decrease granularity.
Problem to be Solved by the Invention
There is a continuing need for methods of improving the speed/fog characteristics of epitaxially sensitized tabular grain emulsions.